Distortion correcting circuit and display device

ABSTRACT

A distortion correcting circuit for correcting vertically asymmetric distortion while achieving reduction in circuit scale and a display apparatus equipped with the circuit are provided. This distortion correcting circuit comprises a comparator circuit  10  which compares an amplitude of an vertical sawtooth wave Vsaw with a reference level Vref which is set to the amplitude of the vertical sawtooth wave corresponding to a center of the screen in a vertical sweeping direction, a multiplier circuit  11  which generates a parabolic wave of the difference between the vertical sawtooth wave and the reference level, a variable gain type amplifier circuit  12  which adjusts the output amplitude of the multiplier circuit  11  and a control circuit  13  which sets a first and second gains of the variable gain type amplifier circuit  12  based upon a result of the comparison in said comparator circuit when a result of the comparison in the comparator circuit  10  represents that the amplitude of the vertical sawtooth wave is lower, and equal to or higher than the reference level, respectively. A distortion correction signal Vsaw is output from the amplifier circuit  12.

FIELD OF THE INVENTION

[0001] The present invention relates to a display apparatus and inparticular to a circuit for correcting pincushion distortion and adisplay device comprising the circuit.

BACKGROUND OF THE INVENTION

[0002] In a cathode ray tube (termed “CRT”) of a Television receiver ordisplay apparatus, as shown in FIG. 16a, while a deflection radius of auniform magnetic field for deflecting an electron beam 73 emitted froman electron gun (cathode electrode 72) defines a spherical face, ascreen (fluorescent surface) 74 is substantially planer so that thedistance from the deflection center to the screen gets longer with theincrease of the deflection angle and the projection size becomes largeras a raster image projected on the screen 74 moves toward the edges ofthe screen 74. The raster image are extended most at the four cornerswhere the deflection angle becomes the largest and are distorted in theform of a pincushion and hence this distortion is termed “pincushiondistortion”. As shown in FIG. 16b, the distortion in which the verticalline at opposing right and left edges on the screen are curved isreferred to as “E-W Raster pincushion distortion” or “Side pincushiondistortion” since it is the distortion at the right and left edges.

[0003] Correction of the left and right pincushion distortion hasheretofore been achieved by causing a horizontal deflection currentwhich is amplitude-modulated with a parabolic current in a verticalscanning period to flow through a horizontal deflection coil.

[0004]FIG. 17 is a block diagram showing an exemplary configuration of aCRT monitor in which a deflection distortion such as a side pincushiondistortion is corrected. Referring to FIG. 17, the CRT monitor comprisesa vertical sawtooth wave oscillating circuit 207 which receives avertical synchronization signal Vsync output from a synchronizationseparating/synchronization input processing circuit 205 and generates avertical sweeping sawtooth wave (termed vertical sawtooth wave) and adeflection distortion correcting circuit 208 which receives the verticalsawtooth wave output from the vertical sawtooth oscillating circuit 207and generates a distortion correction signal Vo made up of a parabolicwave of one vertical sweeping period. The distortion correction signalVo which is generated by the deflection distortion correcting circuit208 is fed to a horizontal oscillating circuit 206 which receives ahorizontal synchronization signal Hsync and generates a horizontalsweeping sawtooth wave (horizontal drive signal). The horizontal drivesignal (the phase of which is controlled by the distortion correctionsignal) which is output from the horizontal oscillating circuit 206 isfed to a horizontal deflection output circuit 210. The horizontal drivesignal output from the horizontal oscillation circuit 206 and thedistortion correction signal Vo output from the deflection distortioncorrection circuit 208 are fed to a +B power supply circuit 209 whichboosts the +B voltage by a pulse amplification in a blanking periodduring which a high voltage is required. As is well known, one cycle ofthe sawtooth wave is made up of a scanning interval during which theelectron beam is scanned at a constant rate and a blanking intervalwhich is a period from the completion of the scanning in one directionto next scanning. The amplitude of the horizontal drive signal withinone vertical period is corrected by correcting +B voltage with thedistortion correction signal Vo, a frequency of which corresponds to onevertical period. The +B voltage is supplied by the +B power supplycircuit 209 to the horizontal deflection output circuit 210 whichprovides a deflection current which flows into a horizontal deflectioncoil. The horizontal deflecting current in which the amplitude of thehorizontal drive signal is changed in proportion with that of thedistortion correction signal is output from the horizontal deflectionoutput circuit 210. The synchronization deflection circuit is configuredas mentioned above. As an output circuit of a video signal (RGB signal),there exists an RGB preamplifier 201 which receives a input video signal(RGB input) and outputs pre-amplified signal to an RGB main amplifier202. The amplified output signal from the RGB main amplifier 202 is fedto an electron gun (not shown) of the CRT 203. In the CRT 203,deflection currents from horizontal and vertical deflection outputcircuits 210 and 211 are supplied to the horizontal and verticaldeflection coils 204, respectively and the electron beam which isemitted from the electron gun is deflected under the magnetic fieldsfrom deflection yokes (not shown) arranged on the neck of the CRT 203.

[0005] A slight variation in a mounting angle of the deflection yoke onthe neck of the CRT 203 in a vertical direction causes a difference in adistortion wave form of a pincushion distortion, trapezoidal distortionand the like between the upper and lower areas of the screen, and hencea vertically asymmetrical distortion is generated.

[0006] Although the raster images at the upper and lower areas of thescreen of the CRT have been corrected by the same amount so that thedistortions are not readily visible, implementation of a feature toindependently correct the side pincushion distortion at upper and lowerareas of the screen has been demanded with a recent increase in a sizeand a planar structure of the screen of CRT.

[0007] Apparatuses for correcting the side pincushion distortion orvertically asymmetric distortion are disclosed in, for example, JapanesePatent Kokai Publication JP-A-6-334887, JP-A-5-308538 andJP-A-11-313222.

[0008] Among of them, in JP-A-6-334887 is disclosed a configuration of acorner pincushion distortion correcting wave generating circuit which iscapable of independently correcting the distortions at corners in theupper and lower levels of the screen. As shown in FIG. 18, thecorrecting wave generating circuit comprises a lower corner partextracting differential comparator circuit 102 which receives a sawtoothwave signal 101 (vertical sawtooth wave signal) having a period of onevertical interval and extracts (slices) a part of the waveformcorresponding to the lower area of the screen in the sawtooth wavesignal 101; a multiplier circuit 105 for multiplying the extractedwaveform; and an amplitude adjusting circuit (gain control circuit) 107for adjusting the amplitude of the corrected wave, whereby theabove-mentioned circuits correct the corner distortion in the lower areaof the screen. The correction wave generating circuit further comprisesan upper corner part extracting differential comparator circuit 103which receives the sawtooth wave signal 101 and extracts a part of thewaveform corresponding to the upper area of the screen in the sawtoothwave signal 101; a multiplier circuit 106 for multiplying the extractedwaveform; and an amplitude adjusting circuit (gain control circuit) 108for adjusting the amplitude of the corrected wave, whereby the circuitscorrect the corner distortion at the upper area of the screen. Thecorrecting wave generating circuit further includes an amplitudeadjusting circuit (gain control circuit) for adjusting the amplitude ofthe signal after the lower corrected wave is added to the uppercorrected wave in an adder 109. A corner slice point bias circuit 104provides reference bias voltages used in the lower and upper cornerimage extracting differential comparator circuits 102 and 103respectively for extracting the lower and upper corner parts in imagesto the lower and upper corner extracting differential comparatorcircuits 102 and 103, respectively.

[0009] In JP-A-5-308538 is disclosed a left and right side pincushiondistortion correcting apparatus for correcting the side pincushiondistortion wherein a current flowing through the horizontal deflectioncoil of a horizontal output circuit is modulated in a vertical period bya parabolic voltage generated by integrating the vertical sawtooth wavevoltage and wherein the apparatus comprises a correction section set upunit for forming a selection signal for each correction section which isobtained by dividing each of a vertical synchronization period withreference to the vertical synchronization signal or horizontalsynchronization signal and a correction quantity adjusting unit whichselects a correction adjusting voltage for each correction section inresponse to a selection signal of each correction section forsuperimposing the correction adjusting voltage of each correctionsection to the parabolic wave voltage. With this apparatus, thecorrection section is obtained by dividing the vertical synchronizationperiod, the correction quantity adjusting voltage is given to eachsection, the correction quantity adjusting voltage in each correctionsection is put on the parabolic voltage, and parabolic wave voltage isgenerated at an integration circuit by integrating the vertical sawtoothwave voltage so that the central position of the amplitude of thevertical sawtooth wave matches the bottom of the parabolic wave voltageand the correction interval is preset with reference to the verticalsynchronization signal and the correction section is set up withreference to the vertical synchronization signal.

[0010] In JP-A-11-313222, is disclosed an image distortion correctingapparatus which reduces distortion which is asymmetric in upper andlower parts of a screen. With this apparatus, an output of the side pincorrecting circuit is a parabolic wave plus a third power wave (S-shapewave). The amplitude of the parabolic wave of the output signal ischanged by varying a side pin control voltage, and hence a pincushiondistortion which is symmetric in left and right sides of the screen canbe corrected. The amplitude of the third power wave is changed byvarying an S-shape distortion control voltage, and hence verticalS-shape distortion which is asymmetric in upper and lower parts of ascreen can be corrected. The correction quantity in the third power waveis automatically changed depending upon the vertical size of the screenand vertical position by using a vertical sawtooth wave includinginformation on the vertical size of the screen and the verticalposition. With this apparatus, the distortion adjustment operation iscomplicated because the operation adjusts two signals (the parabolicwave and the 3rd power wave) for the distortion of the whole screen tobe minimized.

SUMMARY OF THE DISCLOSURE

[0011] As mentioned in the above, the apparatus which is disclosed inJP-A-6-334887 has two systems each comprising a differential comparatorcircuit, multiplier circuit and amplitude adjusting circuit for each ofthe upper and lower parts of a screen in order to control independentlythe upper and lower parts of the screen. Accordingly, the apparatus hasa problem that the circuit scale increases. Parts in a vertical sawtoothwave corresponding to a lower part (lower corner) and an upper part(upper corner) of the screen are respectively extracted by thedifferential comparator circuits 102 and 103 and are respectivelymultiplied by the multiplier circuits 105 and 106. If there arevariations in the offsets of two differential comparator circuits 102and 103, the variations in the offsets are also multiplied by themultiplier circuits 105 and 106. The correction signal which is obtainedby adding the lower waveform to the upper waveform may have an offset(step) due to the difference between the offsets in the upper and loweradjusting circuits. When the gains of two amplitude adjusting circuits107 and 108 are changed, the offset voltage are changed, which maychange the raster image size.

[0012] It is an object of the present invention to provide a distortioncorrecting circuit and a display apparatus in which a distortion isindividually corrected for each one in the upper and lower parts of ascreen while reducing a circuit scale.

[0013] Another object of the present invention is to provide adistortion correcting circuit and a display apparatus which is capableof correcting properly distortions in a lower and upper areas of ascreen which are asymmetric even when a center of a vertical sawtoothwave does not correspond to a center of the screen.

[0014] At least one of the above mentioned objects is accomplished by adistortion correcting circuit in accordance with one aspect of thepresent invention which comprises: comparing means for comparing anamplitude of a sawtooth wave signal used for a vertical sweep (termed“vertical sawtooth wave”) with one predetermined reference level or aplurality of predetermined reference levels to detect which part of ascreen in a vertical sweeping direction a currently sweeping position islocated; amplifying means for adjusting an amplitude of a sidepincushion distortion correction signal generated from said verticalsawtooth wave; and control means for individually setting a gain foreach of partial regions of the screen to said amplifying means on thebasis of a detection result by said comparing means.

[0015] In accordance with another aspect of the present invention isprovided a distortion correcting circuit-comprising comparing means forcomparing an amplitude of a vertical sawtooth wave with a predeterminedreference level to detect which of an upper and lower areas of a screena current sweeping position is located at; and control means forindividually setting for the upper and lower areas of the screen thegain of amplifying means which amplifies the amplitude of a sidepincushion distortion correction signal generated from said verticalsawtooth wave based upon a result of the detection conducted by saidcomparing means.

[0016] More specifically, a distortion correcting circuit in accordancewith another aspect of the present invention comprises: a comparatorcircuit which receives a sawtooth wave signal used for a vertical sweep(termed “vertical sawtooth wave”) and compares an amplitude of saidvertical sawtooth wave in a scanning period with one reference level ora plurality of different reference levels, each of which is setrespectively to an amplitude value of the vertical sawtooth wavecorresponding to each predetermined position of the screen in a verticalsweeping direction; a variable gain type amplifier circuit which adjustsan amplitude of an output signal from a multiplier circuit forgenerating a parabolic wave; and a control circuit which alternativelyselects one of a plurality of gains to provide the selected gain to saidvariable gain type amplifier circuit on the basis of a comparison resultby said comparator circuit depending upon which of regions divided bysaid reference level the amplitude of said vertical sawtooth wave islocated, wherein a distortion correction signal is output from saidvariable gain type amplifier circuit.

[0017] A distortion correcting circuit in accordance with another aspectof the present invention comprises: a comparator circuit which receivesa sawtooth wave signal used for a vertical sweep (termed “verticalsawtooth wave”) and compares the voltage of said vertical sawtooth wavein a scanning period with a reference level or a plurality of referencelevels, each of which is set to an amplitude of the vertical sawtoothwave corresponding to a predetermined vertical position of a screen; avariable gain type amplifier circuit which amplifies said verticalsawtooth wave; a control circuit which selects one of plurality ofpredetermined gains depending upon which of regions divided by saidreference level the amplitude of said vertical sawtooth wave is locatedon the basis of a comparison result by said comparator circuit andprovides the gain selected to the variable gain type amplifier circuit;and a multiplier circuit which generates a parabolic wave from an outputsignal output from said variable gain type amplifier circuit to outputsaid parabolic wave as a distortion correction signal

[0018] Still other objects and advantages of the present invention willbecome readily apparent to those skilled in this art from the followingdetailed description, wherein only the preferred embodiment of theinvention is shown and described, simply by way of illustration of thebest mode contemplated of carrying out this invention. As will berealized, the invention is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, all without departing from the invention. Accordingly, thedrawing and description are to be regarded as illustrative in nature,and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a diagram showing the configuration of the firstembodiment of the present invention;

[0020]FIG. 2 is a diagram showing the configuration of the secondembodiment of the present invention;

[0021]FIG. 3 is a diagram showing a modification of the secondembodiment of the present invention;

[0022]FIG. 4 is a circuit diagram showing an exemplary multipliercircuit;

[0023]FIG. 5 is a diagram showing the configuration of the thirdembodiment of the present invention;

[0024]FIG. 6 is a diagram showing the configuration of the fourthembodiment of the present invention;

[0025]FIG. 7 is a circuit diagram showing an example of the detailedconfiguration of the first embodiment of the present invention;

[0026]FIG. 8 is a circuit diagram showing an example of the detailedconfiguration of the second embodiment of the present invention;

[0027]FIGS. 9a to 9 d are views for explaining the principle of theoperation of the embodiment of the present invention;

[0028]FIGS. 10a to 10 d are views for explaining the principle of theoperation of the embodiment of the present invention;

[0029]FIGS. 11a and 11 b are graphs for explaining the offset of thedistortion correction signal in the embodiments of the presentinvention;

[0030]FIGS. 12a and 12 b are graphs for explaining the offset of thedistortion correction signal in a prior art as a reference case;

[0031]FIG. 13 is a graph for explaining the distortion correction signalin another prior art as a reference case;

[0032]FIGS. 14a to 14 f are views for explaining the distortioncorrecting operation in the embodiments of the present invention;

[0033]FIGS. 15a to 15 f are views for explaining the distortioncorrecting operation in a prior art as a control;

[0034]FIGS. 16a and 16 b are diagrams for explaining the pincushiondistortion;

[0035]FIG. 17 is a block diagram showing the basic structure of a CRTmonitor including the distortion correcting circuit;

[0036]FIG. 18 is a block diagram showing the configuration which isdisclosed in JP-A-6-334887.

PREFERRED EMBODIMENTS OF THE INVENTION

[0037] The embodiments of the present invention are now described in thebelow. In the present invention, a partial area in which a currentsweeping position is located is detected by comparing an amplitude of avertical sawtooth wave with one reference level or a plurality ofreference levels while a plurality of partial areas (such as an upperand lower parts) are sectioned and located along a vertical sweepingdirection on the screen, and an amplitude of the distortion correctionsignal is individually controlled for each of the partial areas on thescreen by variably controlling a gain of an amplifier circuit whichadjusts the amplitude of a left and right pincushion distortion (termedside pincushion distortion) correction signal (parabolic wave) for eachof the partial areas of the screen on the basis of the detection result.The apparatus is configured so that the amplitude of the side pincushiondistortion correction signal is smoothly continued between the partialareas of the screen without any noticeable step. For example, theamplitude of the side pincushion distortion correction signal issmoothly continued at the center of the screen in case of partial areasbeing made up of the upper and lower areas of the screen.

[0038] Referring to FIG. 1, a distortion correcting circuit inaccordance with an embodiment of the present invention comprises acomparator circuit 10 which receives as an input a vertical sawtoothwave and compares an amplitude of the vertical sawtooth wave (theamplitude of the vertical sawtooth wave in a scanning period) with areference level which is set to an amplitude of the vertical sawtoothwave corresponding to a center position of the screen along a verticalsweeping direction. If the voltage amplitude of the vertical sawtoothwave are to be compared, the reference voltage Vref is provided to thecomparator circuit 10 as the reference level . The distortion correctingcircuit further comprises a multiplier circuit 1 1 which receivesvertical sawtooth wave and the reference level and generates a parabolicwave of a difference between the input vertical sawtooth wave and thereference level, a variable gain amplifier 12 which adjusts an outputamplitude of the multiplier circuit 11 and a control circuit 13 whichprovides, based on the comparison result by the comparator circuit 10, afirst gain A1 to the variable gain type amplifier circuit 12 when acomparison result by the comparator circuit 10 indicates that theamplitude of the vertical sawtooth wave is lower than the referencelevel and a second gain A2 to the variable gain type amplifier circuit12 when a comparison result by the comparator circuit 10 indicates thatthe amplitude of the vertical sawtooth wave is higher than or equal tothe reference level. A distortion correction signal Vo is output from anoutput terminal of the amplifier circuit 12.

[0039] Referring now to FIG. 2, a distortion correcting circuit inaccordance with another embodiment of the present invention comprises acomparator circuit 10 which compares an amplitude of the verticalsawtooth wave signal with a reference level which is set to theamplitude of the vertical sawtooth wave corresponding to a center of thescreen in a vertical sweeping direction, a variable gain amplifiercircuit 12 which amplifies a difference between the vertical sawtoothwave and the reference level, a control circuit 13 which selects andprovides, based on the comparison result by the comparator circuit 10, afirst gain to the variable gain type amplifier 12 when the amplitude ofthe vertical sawtooth wave is lower than the reference level and selectsand provides a second gain to the variable gain type amplifier 12 whenthe amplitude of the vertical sawtooth wave is higher than or equal tothe reference level, a multiplier circuit 11 which generates a parabolicwave from the output from the variable gain amplifier circuit 12 and thedifference between the vertical sawtooth and the reference voltage andoutputs it as a distortion correction signal Vo.

[0040] The embodiments of the present invention will be described indetail with reference to drawings. FIG. 1 illustrates a schematicdiagram of a distortion correcting circuit in accordance with a firstembodiment of the present invention. Referring to FIG. 1, the distortioncorrecting circuit of the first embodiment of the present inventioncomprises a comparator circuit 10 which receives as an input a verticalsawtooth wave Vsaw generated by the sawtooth wave generating circuit ofthe vertical output unit (not shown) and compares the voltage of thevertical sawtooth wave in a scanning period with a reference voltageVref which is set to the voltage of the vertical sawtooth wavecorresponding to a center position of the screen in a vertical sweepingdirection, a multiplier circuit 11 which receives the vertical sawtoothwave Vsaw and reference voltage Vref and generates a parabolic wave ofthe difference between the vertical sawtooth wave Vsaw and the referencevoltage Vref (Vsaw−Vref), a variable gain type amplifier circuit 12which adjusts an amplitude of the output of the multiplier circuit 11,and a control circuit 13 which receives a comparison result by thecomparator circuit 10 as a selection control signal and sets the gain ofthe amplifier circuit 12 to a first gain A1 when the voltage of thevertical sawtooth wave is lower than the reference voltage Vref, and toa second gain A2 when the voltage of the vertical sawtooth wave is equalto or higher than the reference voltage Vref, respectively. A distortioncorrection signal Vo having a period equal to one vertical period isoutput from the output terminal of the amplifier circuit 12. Theamplitude of the distortion correction signal Vo is adjusted by theamplifier circuit 12 with gains which are different at a first half anda latter half intervals of one vertical period.

[0041] The amplifier circuit 12 which adjusts the amplitude of theoutput of the multiplier circuit 11 is comprised of a voltage controlledamplifier, the gain of which is varied based on a control voltage whichis fed to the amplifier circuit 12. In this case, the control circuit 13selects a control voltage Vc1 corresponding to the first gain A1 and acontrol voltage Vc2 corresponding to the second gain A2 on the basis ofa comparison result by the comparator circuit 10 and provides theselected control voltage to the voltage controlled amplifier circuit 12.

[0042] The reference voltage Vref which is fed to the comparator circuit10 is set to a voltage of the vertical sawtooth wave corresponding tothe center position of the screen in a vertical sweeping direction. Thecomparator circuit 10 is made up of a voltage comparator. Alternatively,the amplitude of the vertical sawtooth wave may be supplied to thecomparator circuit 10 as an current and is converted to a voltage whichis compared with the reference voltage. Alternatively, the current maybe compared with the current corresponding to the reference voltage.

[0043] A bottom of a center of the distortion correction signal(parabolic wave) is located at a center position of the screen as shownin FIG. 10c as also in the case in which the center P of the screen isshifted from the center Q of the vertical sawtooth wave as shown inFIGS. 10a and 10 b. This shift is generated, for example by variationsin mounting of deflecting yoke in a manufacturing process. Thedistortion correction signal can be generated, which is capable ofproperly correcting the side pincushion distortion which is asymmetricin a vertical direction on the screen.

[0044] As for a reference case in which the reference voltage Vref isset to the center of the amplitude of the vertical sawtooth wave, ashift of a center of the screen from a center of the amplitude of thevertical sawtooth wave makes the distortion correction signal such oneas shown by a dotted line in FIG. 10c and the bottom of the center ofthe distortion correction signal (parabolic wave) is not located in thecenter position of the screen. Accordingly, the distortion which dependson the position on the screen can not be properly corrected. Such aproblem occurs in, for example, an apparatus which is disclosed inabove-mentioned JP-A-5-308538. Since the parabolic wave is not generatedby the multiplier circuit, but is generated by an integrator circuit inthe apparatus which is disclosed in JP-A-5-308538 as mentioned above,the center of the amplitude of the vertical sawtooth wave matches thebottom of the center of the distortion correction signal so that thedistortion correction signal is such one as shown by the dotted line inFIG. 10c even if the center of the vertical sawtooth wave is shiftedfrom the center of the screen. With an apparatus disclosed in theabove-mentioned JP-A-5-308538, in which the correction section in whichthe gain is changed is set with reference to the verticalsynchronization signal, if the center of the amplitude of the verticalsawtooth wave is shifted from the center of the screen in a verticaldirection after the adjustment of the distortion, the distortioncorrection will become ineffective and hence it is necessary to performa distortion adjustment again, for example, by changing the relationbetween the correction section and the correction amount. In contrast tothis, the distortion correction signal is generated by using thereference voltage which is set to a voltage corresponding to the centralposition of the screen in the present embodiment, so that the bottom ofthe center of the distortion correction signal constantly coincides withthe center of the screen. As a result, even if the center of thedisplayed image is shifted upwardly or downwardly on the screen afterthe distortion adjustment is conducted once, no readjustment of the sidepincushion distortion is necessitated.

[0045] A second embodiment of the present invention will now bedescribed. FIG. 2 is a diagram illustrating a configuration of adistortion correcting circuit which forms the second embodiment of thepresent invention. Referring to FIG. 2, the distortion correctingcircuit in accordance with the second embodiment of the presentinvention is different from that of the first embodiment in thearrangement of the multiplier circuit and the variable gain typeamplifier circuit. More specifically, the distortion correcting circuitof the second embodiment comprises a comparator circuit 10 whichreceives a vertical sweeping vertical sawtooth wave Vsaw as an input andcompares the voltage of the vertical sawtooth wave Vsaw with a referencevoltage Vref which is set to the voltage of the vertical sawtooth wavecorresponding to a center position of the screen in a vertical sweepingdirection, a variable gain type amplifier circuit 12 which amplifies adifference voltage between the voltage of the vertical sawtooth waveVsaw and the reference voltage Vref, a control circuit 13 which receivesa comparison result by the comparator circuit 10 as a selection controlsignal and sets a gain of the amplifier circuit 12 to a first gain A1when the voltage of the vertical sawtooth Vsaw is lower then thereference voltage, and to a second gain A2 when the voltage of thevertical sawtooth Vsaw is equal or higher than the reference voltageVref, respectively, and a multiplier circuit 11 which generates aparabolic wave by using the output from the amplifier circuit 12 tooutput the parabolic wave as a distortion correction signal Vo. As isthe case of the first embodiment, the amplifier circuit 12 may beconstituted of a voltage controlled amplifier circuit (VCA) having again which is varied on the basis of a control voltage fed to theamplifier. In this case, the control circuit 13 provides controlvoltages Vc1 and Vc2 corresponding to the first and second gains A1 andA2, respectively to the voltage controlled amplifier circuit 12 basedupon a comparison result in the comparator circuit 10.

[0046] The multiplier circuit 11 outputs the distortion correctionsignal Vo having an amplitude corresponding to a value (A×(Vsaw−Vref)²)which corresponds to an output from the amplifier circuit 12 having again A (A×(Vsaw−Vref)) which is multiplied by the difference between thevertical sawtooth wave Vsaw and reference voltage Vref (Vsaw−Vref).

[0047] Any known circuit may be utilized as the multiplier circuit 11.Connections of signals in the multiplier circuit 11 as shown in FIG. 2will now be described with reference to a Gilbert multiplier circuitwhich outputs a value corresponding to a product of the input voltages Xand Y. FIG. 4 is a diagram of a basic configuration of the Gilbertmultiplier circuit. Referring to FIG. 4, this multiplier circuitcomprises a first differential pair of transistors Q1, Q2, a constantcurrent source I₀, and second and third differential pairs oftransistors Q3, Q4 and Q5, Q6. Emitters of transistors Q1 and Q2 arecommonly connected to the constant current source I₀, emitters oftransistors Q3 and Q4 are commonly connected to a collector of thetransistor Q1, and emitters of transistors Q5 and Q6 are commonlyconnected to a collector of the transistor Q2. Collectors of transistorsQ3 and Q4 and collectors of transistors Q5 and Q6 are mutuallycross-connected. A first voltage X is fed differentially to differentialinput terminals A+ and A− which are connected to the bases of the seconddifferential transistor pairs Q3, Q4 and third differential transistorpairs Q6 and Q5. A second voltage Y is fed differentially todifferential input terminals B+, B− which are connected to the bases ofthe first differential transistor pair Q1 and Q2. A current which isproportional to a value X×Y which is the first voltage X multiplied withthe second voltage Y is output as a differential current ΔI(=I₁−I₂)between the collector currents of the transistors Q3 and Q6. Input andoutput terminals of a current mirror current (not shown) are connectedto the collectors of the transistors Q3, Q6, respectively, so that adifferential current ΔI between the collector currents of thetransistors Q3 and Q6 are taken out from the connection points of theoutput terminal of the current mirror circuit and the collector of thetransistor Q6. This differential current ΔI is used if the multipliercircuit 11 outputs the distortion correction signal (parabolic wave). Avoltage corresponding to a value X×Y is output by converting thedifferential current ΔI to a voltage by a current-voltage convertercircuit (not shown) if the multiplier circuit 11 outputs the distortioncorrection signal Vo as a voltage.

[0048] In the present embodiment, the outputs (differential outputs) ofthe amplifier circuit 12 are connected to the differential terminals B+,B− of the multiplier circuit 11, the vertical sawtooth wave Vsaw is fedto the input terminals A+ and the reference voltage Vref is fed to theinput terminal A−. In this case, the distortion correcting signal Vohaving an amplitude corresponding to a value {A×(Vsaw−Vref)}×(Vsaw−Vref)is output from the multiplier circuit 11.

[0049] In this embodiment, as is the case with the first embodiment,even if the center of the screen is shifted from the center of thevertical sawtooth wave, the bottom of the center of the distortioncorrection signal (parabolic wave) is located at the center of thescreen, and hence the distortion correction signal which is capable ofproperly correcting the side pincushion distortion which is asymmetricwith respect to upper and lower areas of the screen.

[0050]FIG. 3 is a diagram showing a modification of the secondembodiment of the present invention. Referring to FIG. 3, thismodification is different from the above mentioned second embodiment inthat a signal fed to the multiplier circuit 11 is only an output signalA×(Vsaw−Vref) from the amplifier circuit 12. The distortion correctionsignal Vo is output having an amplitude corresponding to a value{A×(Vsaw−Vref)}×{A×(Vsaw−Vref)} from the multiplier circuit 11.

[0051] In the modification, outputs (differential outputs) of theamplifier circuit 12 is connected to differential input terminals B+, B−of the amplifier circuit 11 as shown in FIG. 4 and is also connected todifferential input terminals A+, A−, so that A²×(Vsaw−Vref)² is outputas the distortion correction signal Vo. The operation and effect of thismodification is identical with that of the second embodiment.

[0052] A third embodiment of the present invention will be described.FIG. 5 is a diagram illustrating the configuration of the distortioncorrecting circuit which is the third embodiment of the invention.Referring now to FIG. 5, the distortion correcting circuit in accordancewith the third embodiment of the present invention comprises acomparator circuit 10A which compares the voltage of the verticalsawtooth wave Vsaw with a plurality of reference voltages Vref1, Vref2,and Vref3 which are set to the amplitudes of the vertical sawtooth wavesrespectively corresponding to predetermined vertical positions(predetermined positions located along a vertical sweeping direction) onthe screen, multiplier circuit 11 which generates a parabolic wave fromthe vertical sawtooth wave, a variable gain type amplifier circuit 12which adjusts the output voltage of the multiplier circuit 11 and acontrol circuit 13.

[0053] The control circuit 13A, based on a detection result by thecomparator circuit 10A what section the voltage of the vertical sawtoothwave Vsaw is located at with regard to a plurality of sections of avertical period, in which these sections are defined by the plurality ofreference voltages Vref1, Vref2, and Vref3, selects a gain correspondingto the section from a plurality of gains A1 to A4 and provides theselected gain to the variable gain type amplifier circuit 12. Thevariable gain type amplifier circuit 12 amplifies the output voltagefrom the multiplier circuit 11 with the gain and outputs the amplifiedsignal from an output terminal as a distortion correction signal.

[0054] The multiplier circuit 11 is constituted to multiply thedifference between the vertical sawtooth wave Vsaw and the referencevoltage Vref. The reference voltage Vref which is fed to the multipliercircuit 11 may be fixed to, for example, the reference voltage Vref 2,or may be selected among the reference voltages Vref1, Vref2, and Vref3depending upon which section of the vertical period the voltage of thevertical sawtooth wave Vsaw is located at and may be supplied from thecomparator circuit 10A to the multiplier circuit 11.

[0055] Referring to FIG. 5, the reference voltages Vref1, Vref2, andVref3 which are fed to the comparator circuit 10A are generated bydividing the constant voltages Vmax, and Vmin by voltage dividingresistors R1, R2, R3,and R4 in FIG. 5. The voltages Vref1, Vref2, andVref3 may be generated respectively by discrete bias voltage sources.The number of the reference voltages may be 2, or more than 3. Thecontrol circuit which receives output (plural bits outputs) from thecomparator circuit 10A and selects a gain among the plurality of gainsA1 to A4 is made up of, for example, a decoder circuit and a change-overswitch (both not shown). The decoder circuit decodes an output resultfrom the comparator circuit 10A. The change-over switch selects one ofthe plurality of gains A1 to A4 at a time depending upon which of thesections divided by the reference voltages Vref1, Vref2, and Vref3, thevoltage of the vertical sawtooth wave Vsaw received is currently locatedat based upon a decoding result from the decoder and outputs theselected gain to the amplifier circuit 12 as the gain A.

[0056] In the third embodiment of the present invention, the referencevoltage Vref2 is for example set to the voltage of the vertical sawtoothwave corresponding to the center of the screen in a vertical sweepingdirection. In FIG. 5, the voltage dividing resistors R1, R2, R3, and R3for generating the reference voltages are comprised of variableresistors, the resistances of which are variable and the resistances ofthe resistors are adjusted on an adjustment process for shipment in amanufacturing process of the display apparatus so that they have desiredresistances. Alternatively, the preset values of the reference voltagesmay be stored in a non-volatile memory such as EEPROM (not shown). Thevalues of the non-volatile memory may be set into a register (not shown)on a power on sequence, so that the reference voltages are output bysetting the values of the reference voltage source (not shown) basedupon the values in the register.

[0057] In this embodiment, as is the case with the first and secondembodiment, even if the center of the screen is shifted from the centerof the vertical sawtooth wave, the bottom of the center of thedistortion correction signal (parabolic wave) is located at the centerof the screen, and hence the distortion correction signal which iscapable of properly correcting the side pincushion distortion which isasymmetric with respect to upper and lower areas of the screen. Sincethe third embodiment is configured in such a manner that the amplitudeof the distortion correction signal is adjusted by a gain correspondingto each of the plurality of sections of the screen in a verticaldirection thereof, the side pincushion distortion can be properlyadjusted for each section.

[0058] Now, a fourth embodiment of the present invention will bedescribed. FIG. 6 is a diagram illustrating the configuration of thedistortion correcting circuit which forms the fourth embodiment of thepresent invention. Referring now to FIG. 6, the distortion correctingcircuit is substantially identical with that in the third embodimentexcept that the arrangement of the amplifier circuit 12 and themultiplier circuit 11 is changed so that the distortion correctionsignal Vo is output from the multiplier circuit 11 which multiplies anoutput signal amplified by the amplifier circuit 12. The output of theamplifier circuit 12 (A×(Vsaw−Vref)) and, the vertical sawtooth waveVsaw and reference voltage Vref are fed to the multiplier circuit 11 asis the case with the second embodiment, and A×(Vsaw−Vref)² is output asthe distortion correction signal Vo.

[0059]FIG. 7 is a circuit diagram showing the detailed configuration ofthe control circuit 13 which provides a control voltage to the amplifiercircuit 12 made up of a voltage controlled amplifier circuit in thefirst embodiment of the present invention.

[0060] Referring now to FIG. 7, the control circuit 13 comprises a firstand second registers 14 and 15 which store digital values correspondingto a first and second control voltages, respectively, an AND circuit 16,an AND circuit 17, 2n-input and n-output OR circuit 18, and a digital toanalog converter (DAC) 19. The AND circuit 16 outputs in parallel n-bitsof signals, each of which is a result of logical AND operation of eachbit of the output (n bits) of the first register 14 and a comparisonresult signal from a comparator circuit 10. The AND circuit 16 outputsthe output signal from the first register 14 and 0 when the comparisonresult output from the comparator circuit 10 is 1 and 0, respectively.The AND circuit 17 outputs in parallel n-bits of signals, each of whichis a result of logical AND operation of each bit output from the secondregister 15 (n bits) and a complimentary signal of a comparison resultfrom the comparator circuit 10. The AND circuit 17 outputs the outputsignal from the second register 15 and 0 when the comparison resultoutput from the comparator circuit 10 is 0 and 1, respectively. The2n-input and n-output OR circuit 18 receives n-bit outputs from the ANDcircuit 16, 17 and outputs in parallel n bits of signals, each of whichis a result of a logical OR operation of corresponding pair of bitsignals. The output signal from the OR circuit 18 is fed to the digitalto analog converter (DAC) 19 and the analog voltage output from thedigital to analog converter 19 is fed to the voltage controlledamplifier circuit 12 as a control signal.

[0061] The configuration which is shown in FIG. 7 is equivalent to thefunction of a selector which comprising n pairs of first and secondtri-state buffers (when one of the buffers is output enabled, the otherbuffer is set in a high impedance state) to which receives correspondingtwo bits from first and second registers 14 and 15. The output enableterminals of the first and second buffers are controlled by the outputand the inverted output of the comparator circuit 10, respectively. Theoutput terminals of the first and second buffers are connected and theconnection node of the output terminals of the first and second buffersis connected to the corresponding input of the digital to analogconverter 19.

[0062] On an inspection or test process for shipment of the displayapparatus in a manufacturing process, the values of the gains to be setin the first and second registers 14, 15 are written in a non-volatilememory such as EEPROM (electrically-erasable and programmable read-onlymemory) provided in the display apparatus. On an initialization processexecuted at the time of power on sequence of the display apparatus, thevalues of the gains are read from the non-volatile memory by a CPU (notshown) and are set into the first and second registers 14, 15.

[0063]FIG. 8 is a diagram showing in detail the configuration of thecontrol circuit for supplying a control voltage to the amplifier circuitwhich is made up of a voltage controlled amplifier circuit in amodification of the above-mentioned second embodiment. Referring now toFIG. 8, the configuration of the control circuit 13 is similar to thecontrol circuit 13 which is shown in FIG. 7.

[0064]FIGS. 9 and 10 are schematic views for explaining the principle ofoperation in the above mentioned embodiments of the present invention.The relation between a raster image displayed on a CRT (FIG. 9a), avertical sawtooth wave (FIG. 9b) and a distortion correction signal(FIG. 9c) when a center of the amplitude of the vertical sawtooth wavecoincides with a center of the screen in a vertical sweeping directionis shown in FIG. 9. The waveforms of the vertical sawtooth wave, and theoutput of the comparator circuit 10, the output of the amplifier circuit12 and the output of the multiplier circuit 11 (distortion correctingsignal) in the circuit shown in FIG. 8 are shown in FIG. 9d. Therelation between the levels of the vertical sawtooth wave and thereference voltage Vref is reversed (the output from the comparatorcircuit 10 is reversed) at the center of the amplitude of the verticalsawtooth wave, so that the output of distortion correction signal of themultiplier circuit 11 which multiplies the output of the amplifiercircuit 12 becomes symmetric with respect to the center of the screen asshown in FIG. 9c.

[0065] The relation between a raster image displayed on the CRT (FIG.10a), the vertical sawtooth wave (FIG. 10b) and the distortioncorrection signal (FIG. 10c) when the center of the amplitude of thevertical sawtooth wave does not coincides with the center of the screenin a vertical sweeping direction and is shifted therefrom in theconfiguration of FIG. 8 is shown in FIG. 10. The waveforms of thevertical sawtooth wave, and the output of the comparator circuit 10, theoutput of the amplifier circuit 12 (output of VCA 12) and the output ofthe multiplier circuit 11 (distortion correction signal) in the circuitshown in FIG. 8 are shown in FIG. 10d. If the center of the amplitude ofthe vertical sawtooth is shifted from the center of the screen which isa vertical sweeping direction, for example, the upper end of a raster(starting end of the scanning) is shifted downwardly and a ratio of atime interval between the raster upper end (scanning starting end) andthe screen center P to a time interval between the screen center P andthe raster lower end (scanning completion end) is r:s (r<s in case shownin FIG. 10), the relation between the waveform Vsaw of the verticalsawtooth wave fed to the comparator circuit 10 and the reference voltageVref will become as shown in FIG. 10d. The reference voltage Vref is setto the voltage (voltage of the vertical sawtooth wave) corresponding tothe screen center P in a vertical scanning direction. In this case, thereference voltage is lower than the voltage of the amplitude center Q ofthe input vertical sawtooth. On the contrary, if the shift between theamplitude center of the vertical sawtooth and the screen center in avertical sweeping direction is reversed to that shown in FIGS. 10a and10 b (r>s in FIG. 10(d)), the reference voltage Vref is higher than thevoltage of the amplitude center Q of the input vertical sawtooth wave.

[0066] The amplifier circuit 12 amplifies the input signal at a firstgain A1 until the voltage of the vertical sawtooth wave exceeds thereference voltage Vref after the voltage Vsaw increases from the scanstart voltage. The output signal of the comparator circuit 10 becomesreversed when the vertical sawtooth wave exceeds the reference voltageVref. Thereafter, the amplifier circuit 12 amplifies the input signal atthe second gain A2 until the completion of scanning, and hence thedistortion correction signal Vo becomes a parabolic wave having itsbottom located at the screen center P.

[0067] The reference voltage Vref is set up to a value corresponding tothe screen center by the adjustment for example on a final test for ashipment in a manufacturing process of the display apparatus. The set upvalue of the reference voltage may be written in a non-volatile memorysuch as EEPROM, and on a power on sequence, the set up value of thereference voltage is set in a register (not shown), from which the valueis fed to a digital/analog converter (not shown) which outputs areference voltage.

[0068]FIG. 11a is a graph illustrating the relation between thedistortion correction signal Vo (the amplitude in one vertical period)and the vertical sawtooth wave (solid line connecting rhombus marks) inthe foregoing first and second embodiments. FIG. 11b is a graphillustrating an enlarged central area in FIG. 11b in detail (an axisordinate represented in unit of 100 mV). Since the multiplication is notindividually performed in upper and lower areas of sweeping in thepresent invention, no step occurs in the parabolic signalVpara=K×(Vsaw+Voff1)² from the multiplier circuit 11 between the upperand lower areas of the screen even if there is an input offset Voff1 ofthe multiplier circuit 11.

[0069] With respect to an input offset of the amplifier circuit 12, thedistortion correction signal Vo output from the amplifier circuit 12becomes Vo=A×(Vpara+Voff2) (wherein A denotes the gain of the amplifiercircuit 12). The gain A is set to different values at first and latterhalves of the vertical period. The distortion correction signal Vowithout offset is represented by a dotted line (no Voff) connectingsquare marks in FIG. 11. The distortion correction signal Vo with offsetis represented by a line (with Voff) connecting marks X.

[0070] It is assumed that the vertical sawtooth wave has an amplitude of±5V, the input offset Voff 1 of the multiplier circuit 11 is −100 mV,and the input offset of the amplifier circuit 12 is 100 mV, both thegains A1, A2 in the first and latter halves of one vertical interval are1, the step at the screen center in the distortion correcting signal V(with Voff) is hardly noticed as shown in FIG. 11.

[0071] As a reference case, the configuration corresponding to thatshown in FIG. 18 in which a slice point is set to the screen center willbe explained. FIG. 12a is a graph showing a relation between thedistortion correcting signal Vo (the amplitude of one vertical period)comprising a parabolic wave and the vertical sawtooth wave (a solid lineconnecting rhombic marks) in the configuration corresponding to thatshown in FIG. 18 (provided that the slice point is located at the screencenter). FIG. 12b is a graph showing the enlarged central area in thescreen in FIG. 12a (an axis of ordinate represented in unit of 100 mV).

[0072] If there should be variations between the output offsets of twodifferential comparator circuit (slice circuits), they are squared bythe multiplier circuit. Taking into account the output offset Voff 1 ofthe differential comparator circuit (slice circuit) , the output of themultiplier circuit will become Vpara=K×(Vsaw+Voff1)². If the inputoffsets of the amplitude circuits (gain control circuits) 107, 108 aredenoted as Voff 2, the distortion signal which is a sum of the outputsof two amplitude adjusting circuits 107 from an adder 109 becomesVo=A×(Vpara+Voff2). If the distortion correction signal Vo is slicedinto two parts corresponding to upper and lower areas of the screen bytwo differential comparator circuits, the signal Vo=A×(Vpara+Voff2) isreplaced with values of the differential comparator circuit andamplitude adjusting circuit in the first and latter halves of onevertical period, respectively.

[0073] If it is assumed that for the vertical sawtooth wave of ±5V, thefirst differential comparator circuit (slice circuit) 102 in FIG. 18 hasan offset of −100 mV, the second differential comparator circuit (slicecircuit) 103 has no offset, the first amplitude adjusting circuit 107has an input offset of 100 mV, the second amplitude adjusting circuit108 has an input offset of 0V, the first and second amplitude adjustingcircuits 107, 108 have a gain of 1, the distortion correcting signalwill have an step of 100 mV in the center thereof as shown in FIG. 12(refer to FIG. 12b). This step is changed by the gain set up to theamplitude adjusting circuit. A step in a horizontal direction occurs inthe raster image as shown in FIG. 15c when a step occurs in thedistortion correction signal.

[0074] In contrast to this, no step appears due to an offset inaccordance with the present invention, so that a side pincushiondistortion can be properly corrected as shown in FIG. 14.

[0075] The characteristics of the distortion correction signal will beconsidered with reference to an apparatus as set forth inJP-A-11-313222. In this apparatus, a parabolic wave (=k1×Vsaw²) and athird power wave (A×K2×Vpara×Vsin) are used. The distortion correctionsignal Vo becomes as shown in FIG. 13. If only the lower area of thescreen is corrected, it suffices to make the distortion correctionsignal for the upper area of the screen 0, but a distortion newly occursin the vicinity of ¼ and ¾ of the screen. In other words, when a thirdpower wave which is symmetric with respect to the center of the screenin a vertical direction and correction of, for example, upper area ofthe screen is conducted, the third power wave gives an influence uponthe parabolic wave in the lower area of the screen if the distortioncorrecting wave is zero at the upper area of the screen.

[0076] In contrast to this, no combination of a parabolic wave with athird power wave is used and the distortion correction is simplified inaccordance with the present invention. The distortion due to the thirdpower wave as in the configuration as set forth in JP-A-11-313222 willnot appear. Accordingly, the adjustment for the distortion is maderemarkably easy.

[0077] The distortion correcting circuit of the present invention may beapplied to any display apparatus including a CRT and is used as thedeflection distortion correcting circuit 108 for the display apparatusshown in FIG. 17. The distortion correction signal having one periodwhich is one vertical period is amplitude-modulated with a horizontaldeflection current and is supplied from the horizontal deflection outputcircuit 210 to a horizontal deflection coil.

[0078] The distortion correcting circuit of the above-mentionedembodiments is preferably applied to the correction of the verticallyasymmetric distortion of the display apparatuses which increases theirsize and flatness of the screen. In comparison with the configurationhaving two systems each comprising a differential comparator circuit,multiplier circuit, amplitude adjusting circuit (voltage controlledamplifier circuit) for each of upper and lower areas of the screen likethe configuration disclosed in the above-mentioned JP-A-6-334887 (referto FIG. 8), only one system comprising a differential comparatorcircuit, multiplier circuit, and amplitude adjusting circuit is requiredin accordance with the present invention. The circuit configuration ofthe control circuit is small in scale as shown in FIGS. 7 and 8. Theoffset in the present invention is remarkably smaller in comparison withthat of the configuration disclosed in JP-A-6-334887, so that no stepappears in the raster image. Reduction in cost and improvement in imagequality can be achieved by a simple circuit configuration.

[0079] Although the present invention has been described with referenceto embodiments, the present invention is not limited to the foregoingembodiments. It is to be understood that the present invention includesany modifications and change made by those skilled in the art withoutdeparture from the spirit and scope of the invention as set forth in theclaims.

[0080] The meritorious effects of the present invention are summarizedas follows.

[0081] As mentioned above, the present invention provides a meritoriouseffect that the distortion of image can be individually corrected ateach areas such as in the upper and lower areas of the screen whilereducing the scale of the circuit. The reason is that in the presentinvention, only one system comprising a multiplier circuit forgenerating a distortion correcting parabolic wave and an amplifiercircuit is required for a plurality of regions which divide the screen.

[0082] The present invention further provides a meritorious effect thata side pincushion distortion and a vertically asymmetric distortion suchas a trapezoidal distortion can be properly corrected even if a centerof a vertical sawtooth wave is shifted from a center of the screen,while the shift for example being caused by variations in mountingposition of an electron gun. The reason is that in the presentinvention, the amplitude of the vertical sawtooth wave is compared withthe reference voltage which is set to an amplitude of the verticalsawtooth wave corresponding to the center of the screen for changing thegain of the amplifier circuit at upper and lower areas of the screen.

[0083] In accordance with the present invention, the distortionadjustment of the display apparatus is simplified by a simple circuitconfiguration, and reduction in manufacturing cost can be achieved. Thepresent invention is preferably applied to the correction of verticallyasymmetric distortion of the display apparatus having a large sizescreen.

[0084] The present invention remarkably reduces an offset at the centralpart of the distortion correction signal (between partial areas), whilethe offset being caused for example by offsets of a multiplier circuitand an amplifier circuit for amplitude adjustment, so that a step isprevented from appearing on the raster image and contributes to animprovement in image quality.

[0085] In accordance with the present invention, the distortioncorrection signal is generated by using a reference voltage which is setto a voltage corresponding to the center position on the screen, so thatthe bottom of the center of the distortion correction signal (parabolicwave) constantly coincides the center position of the screen.Accordingly, necessity of readjustment for the side pincushiondistortion is unnecessary, and an adjustment process becomes remarkablyeasy even if the center of the amplitude of the vertical sawtooth waveis shifted from the center of the screen after the adjustment fordistortion has been conducted once.

[0086] It should be noted that other objects, features and aspects ofthe present invention will become apparent in the entire disclosure andthat modifications may be done without departing the gist and scope ofthe present invention as disclosed herein and claimed as appendedherewith.

[0087] Also it should be noted that any combination of the disclosedand/or claimed elements, matters and/or items may fall under themodifications aforementioned.

What is claimed is:
 1. A distortion correcting circuit comprising:comparing means for comparing an amplitude of a sawtooth wave signalused for a vertical sweep (termed “vertical sawtooth wave”) with onepredetermined reference level or a plurality of predetermined referencelevels to detect which part of a screen in a vertical sweeping directiona currently sweeping position is located; amplifying means for adjustingan amplitude of a side pincushion distortion correction signal generatedfrom said vertical sawtooth wave; and control means for individuallysetting a gain for each of partial regions of the screen to saidamplifying means on the basis of a detection result by said comparingmeans.
 2. A distortion correcting circuit comprising: comparing meansfor comparing an amplitude of a sawtooth wave signal used for a verticalsweep (termed “vertical sawtooth wave”) with a predetermined referencelevel to detect which of an upper and lower areas of a screen acurrently sweeping position is located at; and amplifying means foradjusting an amplitude of a side pincushion distortion correction signalgenerated from said vertical sawtooth wave; and control means forindividually setting a gain for each of an upper and lower areas of thescreen to said amplifying means on the basis of a detection result bysaid comparing means.
 3. A distortion correcting circuit comprising: acomparator circuit which receives a sawtooth wave signal used for avertical sweep (termed “vertical sawtooth wave”) and compares anamplitude of said vertical sawtooth wave in a scanning period with onereference level or a plurality of different reference levels, each ofwhich is set respectively to an amplitude value of the vertical sawtoothwave corresponding to each predetermined position of the screen in avertical sweeping direction; a variable gain type amplifier circuitwhich adjusts an amplitude of an output signal from a multiplier circuitfor generating a parabolic wave; and a control circuit whichalternatively selects one of a plurality of gains to provide theselected gain to said variable gain type amplifier circuit on the basisof a comparison result by said comparator circuit depending upon whichof regions divided by said reference level the amplitude of saidvertical sawtooth wave is located, wherein a distortion correctionsignal is output from said variable gain type amplifier circuit.
 4. Adistortion correcting circuit comprising: a comparator circuit whichreceives a sawtooth wave signal used for a vertical sweep (termed“vertical sawtooth wave”) and compares a voltage of said verticalsawtooth wave in a scanning period with a reference voltage which is setto an amplitude of the vertical sawtooth wave corresponding to a centerof a screen in a vertical sweeping direction; a multiplier circuit whichreceives said vertical sawtooth wave and said reference voltage asinputs and generates a parabolic wave of a difference between thevoltage of said vertical sawtooth wave and said reference voltage; avariable gain type amplifier circuit which adjusts an amplitude of anoutput signal output from said multiplier circuit; and a control circuitwhich provides, on the basis of a comparison result by said comparatorcircuit, a first and second predetermined gains to said variable gaintype amplifier circuit when the voltage of said vertical sawtooth wavesignal is lower, and equal to or higher than said reference voltage,respectively, wherein a distortion correction signal is output from saidvariable gain type amplifier circuit.
 5. A distortion correcting circuitcomprising: a comparator circuit which receives a sawtooth wave signalused for a vertical sweep (termed “vertical sawtooth wave”) and comparesa voltage of said vertical sawtooth wave in a scanning period with areference voltage which is set to an amplitude of the vertical sawtoothwave corresponding to a center of a screen in a vertical sweepingdirection; a multiplier circuit which generates a parabolic wave of adifference between the voltage of said vertical sawtooth wave and saidreference voltage; a voltage controlled amplifier circuit which receivesa control voltage, a gain of said amplifier circuit being changed bysaid control voltage, and said parabolic wave to amplify the voltage ofthe said parabolic wave at the gain specified by the control voltage andoutput the amplified parabolic wave as a distortion correction signal;and a control circuit which provides, on the basis of a comparisonresult by said comparator circuit, a first predetermined control voltageto said variable gain type amplifier circuit when the voltage of saidvertical sawtooth wave signal is lower and provides a secondpredetermined control voltage to said variable gain type amplifiercircuit when the voltage of said vertical sawtooth wave signal is equalto or higher than said reference voltage.
 6. A distortion correctingcircuit comprising: a comparator circuit which receives a sawtooth wavesignal used for a vertical sweep (termed “vertical sawtooth wave”) andcompares the voltage of said vertical sawtooth wave in a scanning periodwith a reference level or a plurality of reference levels, each of whichis set to an amplitude of the vertical sawtooth wave corresponding to apredetermined vertical position of a screen; a variable gain typeamplifier circuit which amplifies said vertical sawtooth wave; a controlcircuit which selects one of plurality of predetermined gains dependingupon which of regions divided by said reference level the amplitude ofsaid vertical sawtooth wave is located on the basis of a comparisonresult by said comparator circuit and provides the gain selected to thevariable gain type amplifier circuit; and a multiplier circuit whichgenerates a parabolic wave from an output signal output from saidvariable gain type amplifier circuit to output said parabolic wave as adistortion correction signal
 7. A distortion correcting circuitcomprising: a comparator circuit which receives a sawtooth wave signalused for a vertical sweep (termed “vertical sawtooth wave”) and comparesthe voltage of said vertical sawtooth wave in a scanning period with areference voltage which is set to an amplitude of the vertical sawtoothwave corresponding to a central position of a screen in a verticalsweeping direction; a variable gain type amplifier circuit whichreceives the sawtooth wave and the reference voltage and amplifies adifference between the sawtooth wave and the reference voltage; acontrol circuit which provides, on the basis of a comparison result bysaid comparator circuit, a first predetermined control voltage to saidvariable gain type amplifier circuit when the voltage of said verticalsawtooth wave signal is lower and provides a second predeterminedcontrol voltage to said variable gain type amplifier circuit when thevoltage of said vertical sawtooth wave signal is equal to or higher thansaid reference voltage; and a multiplier circuit which receives anoutput signal from said variable gain type amplifier circuit andgenerates a parabolic wave from the output signal output from saidvariable gain type amplifier circuit to output the parabolic wave as adistortion correction signal
 8. A distortion correcting circuitcomprising: a comparator circuit which receives a sawtooth wave signalused for a vertical sweep (termed “vertical sawtooth wave”) and comparesthe voltage of said vertical sawtooth wave in a scanning period with areference voltage which is set to an amplitude of the vertical sawtoothwave corresponding to a central position of a screen in a verticalsweeping direction; a voltage controlled amplifier circuit which has again changed by a supplied control voltage and amplifies a differencebetween said sawtooth wave and said reference voltage with the gain; acontrol circuit which provides, on the basis of a comparison result bythe comparator circuit, a first predetermined control voltage to saidvoltage controlled amplifier circuit when the voltage of said verticalsawtooth wave signal is lower and provides a second predeterminedcontrol voltage to said voltage controlled amplifier circuit when thevoltage of said vertical sawtooth wave signal is equal to or higher thansaid reference voltage; and a multiplier circuit which generates aparabolic wave from an output signal from said voltage controlledamplifier circuit to output said parabolic wave as a distortioncorrection signal .
 9. The distortion correcting circuit as defined inclaim 5 comprising: a first and second registers for storing thereindigital values corresponding to said first and second control voltages;a selector to which receives the outputs from said first and secondregisters and a signal indicating the comparison result by saidcomparator circuit as a selection control signal, selects and outputsoutput values of the first registers when said comparison result islower than said reference voltage and selects and outputs output valuesof the second registers when said comparison result is equal to orhigher than said reference voltage; and a digital to analog converterwhich receives the output signal from said selector and converts theoutput signal to an analog signal to output said analog signal, whereinthe output of said digital to analog converter is fed to said voltagecontrolled amplifier circuit as a control voltage.
 10. The distortioncorrecting circuit as defined in claim 8 comprising: a first and secondregisters for storing therein digital values corresponding to said firstand second control voltages; a selector to which receives the outputsfrom said first and second registers and a signal indicating thecomparison result by said comparator circuit as a selection controlsignal, selects and outputs output values of the first registers whensaid comparison result is lower than said reference voltage and selectsand outputs output values of the second registers when said comparisonresult is equal to or higher than said reference voltage; and a digitalto analog converter which receives the output signal from said selectorand converts the output signal to an analog signal to output said analogsignal, wherein the output of said digital to analog converter is fed tosaid voltage controlled amplifier circuit as a control voltage.
 11. Thedistortion correcting circuit as defined in claim 9 wherein saidselector comprising: a first logical AND circuit which receives anoutput of said first register and a signal indicating the comparisonresult by said comparator circuit, and outputs a logical AND of eachoutput bit of said first register and said signal indicating thecomparison result; a second logical AND circuit which receives an outputof said second register and a signal which is obtained by inverting thecomparison result signal output from said comparator circuit by aninverter, and outputs a logical AND of each output bit of said secondregister and said reversed comparison result signal; and a logical ORcircuit which receives an output of said first logical AND circuit andan output of said second logical AND circuit and outputs a signalrepresenting a logical OR of the output bit of said first logical ANDcircuit and an output bit of said second logical AND circuitcorresponding to said output bit of said first logical AND bit.
 12. Thedistortion correcting circuit as defined in claim 10 wherein saidselector comprising: a first logical AND circuit which receives anoutput of said first register and a signal indicating the comparisonresult by said comparator circuit, and outputs a logical AND of eachoutput bit of said first register and said signal indicating thecomparison result; a second logical AND circuit which receives an outputof said second register and a signal which is obtained by inverting thecomparison result signal output from said comparator circuit by aninverter, and outputs a logical AND of each output bit of said secondregister and said reversed comparison result signal; and a logical ORcircuit which receives an output of said first logical AND circuit andan output of said second logical AND circuit and outputs a signalrepresenting a logical OR of the output bit of said first logical ANDcircuit and an output bit of said second logical AND circuitcorresponding to said output bit of said first logical AND bit.
 13. Thedistortion correcting circuit as defined in claim 7 wherein saidmultiplier circuit generates said parabolic wave from the differencebetween said vertical sawtooth wave and said reference voltage, and asignal output from said variable gain type amplifier circuit.
 14. Thedistortion correcting circuit as defined in claim 8 wherein saidmultiplier circuit generates said parabolic wave from the differencebetween said vertical sawtooth wave and said reference voltage, and asignal output from said voltage controlled amplifier circuit.
 15. Adisplay apparatus comprising the distortion correcting circuit asdefined in claim 1, wherein a horizontal deflection coil provided insaid display apparatus is driven with a horizontal deflection current,which is amplitude-modulated with a distortion correction signal havinga vertical interval period output from said distortion correctingcircuit.
 16. A display apparatus comprising the distortion correctingcircuit as defined in claim 9 and a non-volatile storage device, whereinvalues to be set in said first and second registers, which are obtainedon an adjustment step in a manufacturing process are stored in saidnon-volatile storage device; and wherein each value which is stored insaid non-volatile storage device is read-out on turning on of saidapparatus and is stored in said first and second registers.